SlimFast was started in 1977 as a product line of the Thompson Medical Company, founded in the 1940s by S. Daniel Abraham. Thompson Medical also sold the controversial weight loss dietary supplement Dexatrim.[6][7] In 1987, Abraham took the brand private, and it was acquired by Unilever in 2000.[8] In 2014, Unilever sold SlimFast to Kainos Capital.[9] After the sale, KSF Acquisition invested with Kainos Capital in order to take responsibility for the SlimFast brand in the UK, Ireland and Germany.[10]

SlimFast used the phrase, "a shake for breakfast, a shake for lunch, then a sensible dinner", for many years to describe the use of the products within the SlimFast plan.[when?][citation needed] With the addition of snacks and an approach that allows for a variety of calorie plans, the brand currently advocates a more flexible system.[citation needed]

SlimFast's product lines span three distinctive eras: those of the original product line (shakes only), the low-carb product line introduced in response to the low-carb diet craze initiated by the Atkins and South Beach Diets, and the simplified "3-2-1" product line introduced in late 2009.

SlimFast was originally just a diet shake product line. It consisted of chocolate, strawberry, and vanilla shakes meant to replace breakfast and lunch. The company suggested customers eat a low-calorie dinner. Usually, dieters would pick a low-calorie frozen dinner brand such as Lean Cuisine or Weight Watchers, as the SlimFast diet was a convenience product line that offered none of its own dinner products. Later, in the mid-1990s, SlimFast began offering meal bars that could be used as meal replacements.[citation needed]

SlimFast struggled, as did many diet foods in 2002, with the rise of low-carbohydrate diets, and from 2002-2003 experienced a 21% drop in sales. The brand responded in 2004 with a line of items designed to compete in the low-carb market as well as introduced SlimFast Optima products, which were lower in sugar than the standard SlimFast products.

Between 2004 and 2009, five types of SlimFast products were available: Original, Optima, Low carb, High protein, and Easy-to-Digest.[11] Optima, the flagship brand, eventually replaced "original recipe" shakes. In addition to having less sugar than the original, SlimFast Optima shakes contain a protein and vegetable fat blend that is claimed to help with hunger control.[12] Optima was available in the same forms as the original products: meal bars, powders, shakes, and snack bars.

In late 2009, SlimFast simplified its product line with the "3-2-1" diet plan. The plan emphasized three 100-calorie snacks, two 200-calorie meal replacements (shakes or meal bars), and one dinner. SlimFast's product line now only consists of 3-2-1 products and no longer includes the Optima, Original, Low-Carb, or Easy-to-Digest versions.

There are two categories of SlimFast products, and two sub-categories each:

Shakes – Both can only be used as meal replacements (the "2" in the 3-2-1 plan means a dieter is allowed two a day; i.e., two shakes, two bars, or one bar and one shake, for a maximum of two "2" items a day).

Ready-to-drink Shakes come in plastic bottles. It is essentially the shake mix mixed with skim milk.

Shake mix powder can be mixed with milk, water, or anything the dieter chooses to mix it with, but fat-free milk is recommended on the container.

Bars – Meal Replacements (item "2") and Snack Bars (item "3")

Meal Replacements are 200 calories and are meant to be used in place of shakes (the "2" in the 3-2-1 plan described above).

Snack Bars are 100-calories and meant to be the "3" in the 3-2-1 plan, as a dieter can have 3 a day. Snack bars are not supplemented with vitamins and minerals.

A dieter could eat two snack bars as their meal replacement, since two snack bars have the same number of calories as one meal bar, although the meal bars are also meant to be rich in vitamins and minerals, whereas the snack bars are only measured by calorie count and not supplemented by vitamins.

SlimFast does not offer products in category "1", which is one "sensible" meal, which the US site suggests should be 500 calories,[13] while the UK site suggests 600 calories.[14]

On December 3, 2009, SlimFast recalled all of its canned products due to possible bacterial contamination.[15] The company stated that it had halted production until the cause was discovered.[16] No further problems or issues have been noted. In 2011, SlimFast stopped producing cans and has since used plastic bottles.[17]